PLC interlocks


Thread Starter


i have a question about plc interlocks ..i suppose to draw the ladder dia for it. but whenever i tried it i am not able to simulate it properly ie, i am able to draw it but the problem is that when i start the compressor with proper interlocks and any of the interlock take place compressor will have to shut down that is working but when i try it for cold water flow low which have a timer in it that timer is not working properly. may be i have done any mistake to connect the timer condition is when cold water flow low comes compressor have to work for 30 sec that is not happenning in my ladder.

the sequence of operation is as shown below .
Compressor Start-Up and Interlocks
1.To start the compressor in ? MANUAL ? mode make the MANUAL switch on. the MANUAL START push button.
3.At this time auxiliary lube oil pump cuts-in which serves to develop lube oil pressure of above 0.8kg./cm2. This pump cuts ? out after 1 minute of pressing MANUAL START push button.
4.When the lube oil pressure is built above 0.8 kg./cm2. and all the other interlocks such as Cooling water flow low, 1st stage discharge temperature high, 2nd stage discharge temperature high are normal then only a Master-relay for Compressor start gets energized.
5.The lube oil pressure should be developed within 10 seconds; otherwise compressor does not start.
6.If any of the above mentioned interlock comes then the compressor should stop running immediately except for cooling water flow low interlock; the compressor should shut down after 10 seconds.
7.Load the compressor by making ?LOAD? switch on. When the LOAD switch is on, the suction valve of the compressor opens and the vent valve closes. In loaded position, compressor allows maximum air to flow to the compressor and closes the passage of venting air from 2nd stage discharge to atmosphere.
8.To unload the compressor, ?UNLOAD? switch is made on. In this position, the loading or suction valve closes; which allows minimum air flow to the compressor and the air is vented from 2nd stage discharge to the atmosphere.
9.To start the compressor in ?AUTO? mode, make the AUTO switch on.
10.In this mode if header pressure is less than 4.8kg/cm2 and allthe interlocks as specified above are normal, then only the Master-relay for compressor start gets energized and the compressor starts
11.In this mode also, auxiliary lube oil pump cuts-in when the Auto switch is on and the pressure of header is less than 4.8kg/cm2. it gets cut-off after 1 minute.
12.The lube oil pressure should reach required value in 10 seconds; otherwise compressor does not start.
13.After any interlock comes, compressor stops immediately. Only for cooling water flow low, compressor stops after 10 seconds.
14.Within 30 seconds after starting the main motor, compressor will load ie. suction valve will open and vent valve closes.

thanx for your help.


Alejandro (Alex) Flores

When you say that you are drawing it. Am I to understand that you are programming the system? What is the plc or software you are using?
i m anand and i m the one who has placed this interlock question and about ur problem that, i m going to program it with the help of ladder logic only, no other computer language
The sequence you have listed seems to be ok (better check out in the field, before attempting the sequence control). Also ensure that all feedback and control signals are properly available at the PLC and in the PLC data base. As you have developed the program in the ladder logic, it should be possible for you test the logic in the off-line (i.e. force all the input signals and verify the proper output signal). Also check out functioning of the timers (very important), independently by enabling them. Any further help in this can be provided only on verifying the make of PLC and the ladder logic which you have developed. Wishing you all the best in testing this.

CS Nagaraj